1. Field of the Invention
The present application relates generally to optical modulators and switches.
2. Description of the Related Art
All-optical fiber modulators and switches are important devices that have been researched for many years mainly because of the desire for low-loss, low-power, fiber-interfaced, optically-addressable switching devices in optical communication and fiber sensor systems. These systems include, but are not limited to, periodic self-healing communication networks, re-configurable optical signal processing, packet switching for local area networks, bit switching, towed sensor arrays, and testing of fiber links.
Unfortunately, very few physical mechanisms are available to modulate the refractive index of a silica fiber in order to induce switching. The widely-studied Kerr effect has an extremely fast response time (e.g., a few femtoseconds) but it is notoriously weak. Kerr-based fiber switches typically utilize power on the order of 20 watts in a 10-meter fiber at 1.55 micrometers for full switching (see, e.g., N. J. Halas, D. Krökel, and D. Grischkowsky, “Ultrafast light-controlled optical-fiber modulator,” Applied Physics Letters, Vol. 50, No. 14, pages 886-888, April 1987; and S. R. Friberg, A. M. Weiner, Y. Silberberg, B. G. Sfez, and P. S. Smith, “Femtosecond switching in a dual-core-fiber nonlinear coupler,” Optics Letters, Vol. 13, No. 10, pages 904-906, October 1988) or a switching power-length product PL of approximately 200 watt-meters. Resonantly-enhanced nonlinearities in fibers doped with a rare earth such as Er3+ are considerably stronger (PL approximately equal to 10−2 watt-meter) but are very slow (e.g., response time of approximately 10 milliseconds; see, e.g., R. A. Betts, T. Tjugiarto, Y. L. Xue, and P. L. Chu, “Nonlinear refractive index in erbium doped optical fiber: theory and experiment,” IEEE Journal of Quantum Electronics, Vol. 27, No. 4, pages 908-913, April 1991; and R. H. Pantell, R. W. Sadowski, M. J. F. Digonnet, and H. J. Shaw, “Laser-diode-pumped nonlinear switch in erbium-doped fiber,” Optics Letters, Vol. 17, No. 4, pages 1026-1028, July 1992). Switching has also been induced thermally in fibers doped with an absorber. For example, a 2.55-centimeter Co2+-doped fiber switch required a switching peak power of 1.8 kilowatts (PL approximately equal to 5 watt-meters), and its response time was approximately 25 nanoseconds (see, e.g., M. K. Davis, and M. J. F. Digonnet, “Nanosecond thermal fiber switch using a Sagnac interferometer,” IEEE Photonics Technology Letters, Vol. 11, No. 10, pages 1256-1258, October 1999).
More recently, Tapalian et al. (H. C. Tapalian, J.-P. Laine, and P. A. Lane, “Thermooptical switches using coated microsphere resonators,” IEEE Photonics Technology Letters, Vol. 14, pages 1118-1120, August 2002) have demonstrated switching in a microsphere resonator coated with an absorbing polymer by shining a 405-nanometer pump beam on the microsphere's surface. The pump heated the polymer and the microsphere, which thermally shifted the microsphere's resonance wavelengths and switched a 1.55-micrometer signal. The use of a resonator greatly reduces the switching power: a pump exposure of only 4.9 milliwatts for approximately 0.5 second was sufficient to shift the resonance by approximately 1,000 linewidths. Since full switching requires a shift of about one linewidth, the switching power was only 4.9 microwatts, and the switching energy of approximately 2.5 microjoules. However, the switch response time was very long (e.g., 0.165 second). Taking the characteristic dimension of such a switch to be the sphere diameter (250 micrometers in this case), this device has a PL product of approximately 1.2×10−9 watt-meter, which is very low. Whispering gallery mode microsphere resonators based on the Kerr effect have also been previously studied (see, e.g., M. Haraguchi, M. Fukui, Y. Tamaki, and T. Okamoto, “Optical switching due to whispering gallery modes in dielectric microspheres coated by a Kerr material,” Journal of Microscopy, Vol. 210, Part 3, pages 229-233, June 2003; A. Chiba, H. Fujiwara, J. Hotta, S. Takeuchi, and K. Sasaki, “Resonant frequency control of a microspherical cavity by temperature adjustment,” Japanese Journal of Applied Physics, Vol. 43, No. 9A, pages 6138-6141, 2004). Compared to other all-optical fiber switches, microsphere-based optical switches offer the unique advantages of extremely small size (e.g., a microsphere is typically only 50-500 micrometers in diameter) and very low switching energy. The reason is that the resonator has such sharp resonances that a very small change in the microsphere index is sufficient to induce full switching.